1. Field of the Invention
The invention relates to a flashed glass consisting of at least two glass layers of different composition, as well as to a method for the manufacture of such a flashed glass.
2. State of the Art
Flashed glasses, for example, the known flashed milk glass, have been industrially manufactured for decades, namely in such a manner that a thin surface layer of cloudy glass is applied to a glass ribbon which is still soft and has just been manufactured in a Fourcault method or in a modified technology, whereupon the two glass layers are allowed to harden into sheet glass in common in a cooling stack.
Phototropic sheet glasses as are described in the German LP No. 21 25 232, whereby please see the German LP No. 15 96 847 as well as the German LP NO. 24 04 752, for example for the composition of phototropic glasses, are suitable for numerous use purposes. For example, there is a considerable need for economical phototropic vitrification elements which are technologically unproblematical to manufacture and are suitable for automobile glazing, skyscraper glazing, for device windows in structures of various types, but are also suitable for opthalmic, cosmetic and medical purposes as well as for various architectural use purposes. It is thereby desirable to be able to vary the phototropy in the greatest possible ranges, without having to influence significant parts of the manufacturing process.
Given previously known phototropic glasses, it has proven difficult to achieve these goals. In particular, the known phototropic glasses are too costly for numerous use purposes due to the relatively high silver concentration in relationship to the glass cross-section. Thereby, the problem lies therein that phototropic sheet glasses, particularly sheet glasses containing silver halogens, require the presence of silver everywhere in the glass over the entire glass cross-section in order to achieve homogenous phototropic properties, although the penetration depth of the excitation radiation usually does not suffice in order to use all of the silver. A further problem lies therein that the standard sheet glass technologies exhibit large glass surfaces of the smelting container, so that easily volatized components such as the silver and the halogens exhibit high evaporation rates. Special smelt container designs, however, are costly and are hitherto hardly in a position to supply phototropic glass panes with sufficient dimensions as are desirec, for example for architectural applications. A further problem lies therein that phototropic glasses usually B.sub.2 O.sub.3 and that, in general, glasses with B.sub.2 O.sub.3 contents of more than 5 weight percent can be manufactured in sheet glass smelting systems and drawing systems only with relative difficulty.